The present invention relates generally to the manufacture of conical face gears and more particularly to a method and apparatus for continuous generation grinding of conical face gears using a worm-shaped grinding wheel and a dressing tool for use on the worm-shaped wheel.
Background Art
Conical face gearing is a fairly recent innovation and consists of a conical involute gear, which serves as the pinion member, and a mating face gear that meshes with the conical involute pinion and satisfies true conjugate action. Conical face gearing offers gear train designers an alternative to spiral bevel gears in large shaft-angle, large reduction ratio angular power transmission applications. Conical face gearing possesses numerous unique features which provide solutions to special applications, such as the adjustment of backlash between a conical involute pinion and a conical face gear without affecting the tooth contact pattern and the conjugate action between the pinion and the conical face gear.
Despite their apparent advantages, conical face gears are not in widespread use, primarily because manufacturing methods for this type of gearing, especially production suitable methods, have heretofore not been developed. Several of the known processes for generating face gears are not readily suitable for the production manufacture of conical face gears. One such process is disclosed in U.S. Pat. No. 5,494,475 entitled xe2x80x9cTool for Producing Crown Wheels and Method for Producing Such a Toolxe2x80x9d, the disclosure of which is hereby incorporated by reference as if fully set forth herein. One disadvantage of this process concerns the geometry of the threaded tool that is employed to form the crown wheel; the ""475 patent defines the threaded tool in a manner such that it has a true involute profile in the normal planar section of the thread. As such, crown wheels formed in this manner are inaccurate due to toe-heel effects that are well known in the industry and are thus unsuitable for demanding applications (e.g., aerospace applications).
Another disadvantage of the ""475 patent concerns the concept of using a series of racks, each of which having a pressure angle from about 5 degrees to about 45 degrees and representing a thin cylindrical layer of the crown wheel. It is well known in the art that standard industrial gears typically have a whole tooth height equal to about 2.25 divided by the diametrical pitch. As such, gear generating tools are required to have a whole tooth height equal to about 2.5 divided by the diametrical pitch to provide the generated gear with sufficient operational clearance to permit meshing engagement with another gear. A rack having a pressure angle of about 40 degrees, however, can have a maximum height of only about 1.872 divided by the diametrical pitch. Accordingly, there are situations in which the rack cannot be used as a generating tool to form a gear or a gear cutting tool that conforms to industry standard tooth heights.
A final concern with the ""475 patent concerns the manner in which a dressing tool is moved in a two-dimensional manner tangentially across the width of the threaded tool. Feeding the dressing tool in this manner will result in interference between the dressing tool and the adjacent tooth when the pressure angle is greater than about 35 degrees. This dressing method is unsuitable for the production of conical face gears having a relatively high degree of accuracy since the cone angle (and possibly a skew angle) in the pinion must be dealt with in three-dimensional space.
Another gear forming method that is specifically designed for conical face gearing is disclosed in commonly assigned U.S. Pat. No. 5,941,124 entitled xe2x80x9cFace Gearing With Conical Involute Pinionxe2x80x9d, the disclosure of which is hereby incorporated by reference as if fully set forth herein. This method utilizes a formed wheel that emulates the action of one tooth of the pinion that is in meshing engagement with the conical face gear. The single-tooth action and the need to make multiple machining passes in forming a single tooth in the conical face gear renders this process extremely slow such that it is not well suited for volume production.
Accordingly, there is a need in the art for a tool and a method for forming a conical face gear which permits highly accurate gears to be produced at a relatively high rate of production.
In one preferred form, the present invention provides a method for dressing a grinding worm that is to be used in forming a conical face gear that meshingly engages a conical involute pinion. The method employs a theoretical conical involute pinion as a reference to orient a dressing tool relative to the grinding worm at an initial position and to control the movement of the dressing tool relative to the grinding worm. A plurality of straight generators that define a reference tooth on the theoretical conical involute pinion and the concept of true conjugate action between the theoretical conical involute pinion and the grinding worm are employed to define a plurality of dressing tool paths that are employed to position and move the dressing tool to form one or more threads in the grinding worm.
In another preferred form, a method for forming a conical face gear that meshingly engages a conical involute pinion is provided. The method employs a theoretical conical involute pinion as a reference for positioning the grinding worm. True conjugate action between the conical face gear and the theoretical conical involute pinion and true conjugate action between the theoretical conical involute pinion and the grinding worm is employed to determine an initial position of a grinding worm relative to the conical face gear and to establish a feed direction for translating the grinding worm across the conical face gear.
A precision grinding apparatus for dressing a grinding worm and forming a conical face gear is also provided.